Exhaust Purification System of Internal Combustion Engine

ABSTRACT

HC feed control for feeding HC into exhaust gas upstream of an SOx trap  11  when a predetermined condition stands is executed. When an SOx trap amount is smaller than a predetermined amount, as HC feed control, first HC feed control feeding HC into the exhaust gas upstream of the SOx trap by a predetermined pattern is executed. When the SOx trap amount is larger than a predetermined amount, as HC feed control, second HC feed control feeding HC into the exhaust gas upstream of the SOx trap by a pattern different from the predetermined pattern, which pattern keeping the temperature of the SOx trap from locally becoming higher than the predetermined temperature or suppressing the formation of a region in the exhaust gas flowing into the SOx trap where the air-fuel ratio becomes locally rich, is executed.

TECHNICAL FIELD

The present invention relates to an exhaust purification system of aninternal combustion engine.

BACKGROUND ART

Japanese Patent Publication (A) No. 6-173652 describes an internalcombustion engine providing an exhaust purification system absorbing NOx(nitrogen oxides) in the exhaust gas in an exhaust passage. Here, theexhaust gas also contains SOx. The NOx absorbent described in JapanesePatent Publication (A) No. 6-173652 absorbs SOx in addition to NOx sothe amount of NOx which the NOx absorbent can absorb ends up beingreduced by exactly the amount of absorption of SOx. Thus, in the exhaustpurification system described in Japanese Patent Publication (A) No.6-173652, an SOx absorbent for absorbing SOx in the exhaust gas isarranged upstream of the NOx absorbent and the SOx absorbent is used toabsorb the SOx in the exhaust gas and prevent SOx from flowing into theNOx absorbent.

DISCLOSURE OF THE INVENTION

In this regard, in general, an SOx absorbent absorbs the SOx in theexhaust gas when the air-fuel ratio of the exhaust gas flowing into itis an air-fuel ratio leaner than the stoichiometric air-fuel ratio andthe temperature of the SOx absorbent is higher than a so-calledactivation temperature. On the other hand, the SOx absorbent releasesSOx when the air-fuel ratio of the exhaust gas flowing into it becomesthe stoichiometric air-fuel ratio or an air-fuel ratio richer than thatand the temperature of the SOx absorbent becomes higher than a certaintemperature higher than the activation temperature (hereinafter referredto as the “SOx release temperature”). Here, the SOx absorbent has theabsorption of SOx in the exhaust gas as its inherent function, so whenthe SOx absorbent should be made to absorb SOx, it is not preferablethat the SOx absorbent end up releasing SOx. Further, this applies notonly to an exhaust purification system provided with an SOx absorbentfor the purpose of absorbing the SOx in the exhaust gas, but alsobroadly to an exhaust purification system provided with an SOx trap forthe purpose of trapping the SOx in the exhaust gas.

An object of the present invention is to reliably prevent an SOx trapfrom ending up releasing SOx when the SOx trap should be made to trapSOx in an internal combustion engine provided with an SOx trap trappingthe SOx in the exhaust gas.

To solve the problem, in a first aspect of the present invention, thereis provided an exhaust purification system of an internal combustionengine providing an SOx trap for trapping the SOx in the exhaust gasinside an exhaust passage, the SOx trap trapping the SOx in the exhaustgas when an air-fuel ratio of the exhaust gas flowing into the SOx trapis an air-fuel ratio leaner than a stoichiometric air-fuel ratio and atemperature of the SOx trap is lower than a predetermined temperatureand releasing the trapped SOx when the air-fuel ratio of the exhaust gasflowing into the SOx trap is the stoichiometric air-fuel ratio or anair-fuel ratio richer than that and the temperature of the SOx trap ishigher than the predetermined temperature and executing HC feed controlfeeding HC into the exhaust gas upstream of the SOx trap when apredetermined condition stands, which exhaust purification system of aninternal combustion engine executes, as the HC feed control, first HCfeed control feeding HC into the exhaust gas upstream of the SOx trap bya predetermined pattern when the amount of SOx which the SOx trap trapsis smaller than a predetermined amount and executes, as the HC feedcontrol, second HC feed control feeding HC into the exhaust gas upstreamof the SOx trap by a pattern different from the predetermined pattern,which pattern keeping the temperature of the SOx trap from locallybecoming higher than the predetermined temperature or suppressing theformation of a region in the exhaust gas flowing into the SOx trap wherethe air-fuel ratio becomes locally rich, when the amount of SOx whichthe SOx trap traps is larger than the predetermined amount.

In a second aspect of the present invention, in the first HC feedcontrol, a predetermined amount of HC is fed into the exhaust gasupstream of the SOx trap per unit time, while in the second HC feedcontrol, an amount of HC smaller than the predetermined amount is fedinto the exhaust gas upstream of the SOx trap per unit time.

In a third aspect of the present invention, in the second HC feedcontrol, HC with a higher diffusion ability into the exhaust gas thanthe HC fed into the exhaust gas upstream of the SOx trap in the first HCfeed control is fed into the exhaust gas upstream of the SOx trap.

In a fourth aspect of the present invention, in the second HC feedcontrol, HC is fed into the exhaust gas upstream of the SOx trap so thata lean degree of the air-fuel ratio of the exhaust gas flowing into theSOx trap is kept larger than a predetermined lean degree.

In a fifth aspect of the present invention, the predetermined leandegree is set larger the lower the temperature of the SOx trap.

In a sixth aspect of the present invention, in the second HC feedcontrol, HC is fed into the exhaust gas upstream of the SOx trap so thatthe amount of local temperature rise of the SOx trap per unit time iskept smaller than the amount of local temperature rise of the SOx trapper unit time allowed in the first HC feed control.

In a seventh aspect of the present invention, in the second HC feedcontrol, HC is fed into the exhaust gas upstream of the SOx trap so thatan amount of temperature rise of the SOx trap as a whole per unit timeis kept smaller than an amount of temperature rise of the SOx trap as awhole per unit time allowed in the first HC feed control.

In an eighth aspect of the present invention, a particulate filtertrapping particulate matter in the exhaust gas is arranged in theexhaust passage downstream of the SOx trap, one predetermined conditionis a fuel removal condition where it is judged if the temperature of theparticulate filter should be raised to a predetermined targettemperature to burn away particulate matter trapped by the particulatefilter, and, when the second HC feed control is executed when theburnaway condition stands, in the second HC feed control, HC is fed intothe exhaust gas upstream of the SOx trap using as a target temperature atemperature lower than the target temperature in the first HC feedcontrol in the case where the first HC feed control is executed when theburnaway condition stands.

In a ninth aspect of the present invention, in the second HC feedcontrol, HC is fed into the exhaust gas upstream of the SOx trap so thata temperature amplitude of the SOx is kept smaller than a temperatureamplitude of the SOx trap allowed in the first HC feed control.

In a 10th aspect of the present invention, an NOx absorbent absorbingthe NOx in the exhaust gas is arranged in the exhaust passage downstreamof the SOx trap, one predetermined condition is an NOx release conditionwhere it is judged that the NOx absorbent should release NOx, and, whenthe second HC feed control is executed when the NOx release conditionstands, in the second HC feed control, HC is fed into the exhaust gasupstream of the SOx trap so that a temperature amplitude of the SOx trapis kept smaller than a temperature amplitude of the SOx trap allowed inthe first HC feed control in the case where the first HC feed control isexecuted when the NOx release condition stands.

In an 11th aspect of the present invention, an oxidation catalystprovided with an oxidizing ability higher than even the oxidizingability of the SOx trap is arranged in the exhaust passage upstream ofthe SOx trap.

Below, the present invention will be able to be understood more clearlyfrom the attached drawings and the description of preferred embodimentsof the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a compression ignition type of internalcombustion engine provided with an exhaust purification system of thepresent invention.

FIGS. 2(A) and (B) are views showing the structure of a particulatefilter.

FIG. 3 is a cross-sectional view of a surface part of a catalyst carrierof an NOx catalyst.

FIG. 4 is a cross-sectional view of a surface part of a catalyst carrierof an SOx trap.

FIGS. 5(A) to (C) are views for explaining NOx release control of anexhaust purification system of a first embodiment.

FIGS. 6(A) to (C) is a view for explaining NOx release control of anexhaust purification system of a second embodiment.

FIG. 7 is a view showing an example of a routine for executing NOxrelease control of an embodiment of the present invention.

FIGS. 8(A) to (C) are views for explaining the PM removal control of theexhaust purification system of a seventh embodiment.

FIGS. 9(A) to (C) are views for explaining the PM removal control of theexhaust purification system of an eighth embodiment.

FIG. 10 is a view showing an example of a routine for executing PMremoval control of an embodiment of the present invention.

FIG. 11 is a view showing an example of a routine for executing NOxrelease control of an exhaust purification system of a 15th embodiment.

FIG. 12 is a view showing an example of a routine for executing PMremoval control of an exhaust purification system of a 16th embodiment.

FIG. 13 is a view showing one of the compression ignition type ofinternal combustion engine to which the present invention can beapplied.

FIG. 14 is a view showing another one of the compression ignition typeof internal combustion engine to which the present invention can beapplied.

FIG. 15 is a view showing still another one of the compression ignitiontype of internal combustion engine to which the present invention can beapplied.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, an embodiment of the present invention will be explained withreference to the drawings. FIG. 1 shows a compression ignition type ofinternal combustion engine provided with an exhaust purification systemof the present invention. In FIG. 1, 1 shows an engine body, 2 acombustion chamber of each cylinder, 3 an electronic control type fuelinjector for injecting fuel into each combustion chamber 2, 4 an intakemanifold, and 5 an exhaust manifold. The intake manifold 4 is connectedthrough an intake duct 6 to an outlet of a compressor 7 a of an exhaustturbocharger 7, while an inlet of the compressor 7 a is connected to anair cleaner 8. Inside the intake duct 6 is arranged a throttle valve 9driven by a step motor. Further, around the intake duct 6 is arranged acooling device 10 for cooling the intake air flowing inside the intakeduct 6. In the embodiment shown in FIG. 1, the engine cooling water isguided into the cooling device 10 where the engine cooling water is usedto cool the intake air. On the other hand, the exhaust manifold 5 isconnected to the inlet of the exhaust turbine 7 b of the exhaustturbocharger 7, while the outlet of the exhaust turbine 7 b is connectedthrough an exhaust pipe 13 to the inlet of the SO_(x) trap 11. Theexhaust pipe 13 has attached to it an HC (hydrocarbon) feed valve 14 forfeeding for example HC in the exhaust gas flowing through the inside ofthe exhaust pipe 13. Further, the outlet of the SOx trap 11 is connectedto the NOx catalyst 12.

The exhaust manifold 5 and the intake manifold 4 are connected to eachother through an exhaust gas recirculation (hereinafter referred to asthe “EGR”) passage 15. Inside the EGR passage 15 is arranged anelectronic control type EGR control valve 16. Further, around the EGRpassage 15 is arranged a cooling device 17 for cooling the EGR gasflowing through the inside of the EGR passage 15. In the embodimentshown in FIG. 1, the engine cooling water is guided inside the coolingdevice 17 where the engine cooling water is used to cool the EGR gas. Onthe other hand, each fuel injector 3 is connected through a fuel feedpipe 18 to a common rail 19. This common rail 19 is supplied inside itwith fuel from an electronic control type variable discharge fuel pump20. The fuel supplied to the inside of the common rail 19 is suppliedthrough the fuel feed pipes 18 to the fuel injectors 3.

An electronic control unit 30 is comprised of a digital computer whichis provided with components connected with each other by abi-directional bus 31 such as a ROM (read only memory) 32, RAM (randomaccess memory) 33, CPU (microprocessor) 34, input port 35, and outputport 36. The SO_(x) trap 11 has a temperature sensor 21 attached to itso as to detect the temperature of the SO_(x) trap 11, while the NOxcatalyst 12 has a temperature sensor 22 attached to it so as to detectthe temperature of the NOx catalyst 12. The output signals of thesetemperature sensors 21 and 22 are input through the corresponding ADconverters 37 to the input port 35. Further, the NOx catalyst 12 has apressure difference sensor 23 attached to it for detecting the pressuredifference before and after the NOx catalyst 12. The output signal ofthis pressure difference sensor 23 is input through the corresponding ADconverter 37 to the input port 35.

An accelerator pedal 40 is connected to a load sensor 41 generating anoutput voltage proportional to the depression amount of the acceleratorpedal 40. The output voltage of the load sensor 41 is input through thecorresponding AD converter 37 to the input port 35. Further, the inputport 35 has a crank angle sensor 42 generating an output pulse everytime the crankshaft rotates by for example 15° connected to it. On theother hand, the output port 36 has the fuel injectors 3, throttle valve9 drive step motor, HC feed valve 14, EGR control valve 16, and fuelpump 20 connected to it through corresponding drive circuits 38.

Next, the NOx catalyst 12 will be explained. The NOx catalyst 12 iscarried on a monolithic carrier of a three-dimensional mesh structure ora pellet-shaped carrier or is carried on a particulate filter forming ahoneycomb structure (hereinafter referred to as “filter”). In this way,the NOx catalyst 12 can be carried on various carriers, but below thecase of carrying the NOx catalyst 12 on a filter will be explained.

FIGS. 2(A) and (B) show the structure of the filter 12 a carrying theNOx catalyst 12. Note that FIG. 2(A) shows a front view of the filter 12a, while FIG. 2(B) shows a side cross-sectional view of the filter 12 a.As shown in FIGS. 2(A) and (B), the filter 12 a forms a honeycombstructure and is provided with a plurality of exhaust flow passages 60,61 extending in parallel with each other. These exhaust flow passagesare comprised of exhaust gas inflow passages 60 with downstream endsclosed by plugs 62 and exhaust gas outflow passages 61 with upstreamends closed by plugs 63. Note that the hatched parts in FIG. 2(A) showthe plugs 63. Therefore, the exhaust gas inflow passages 60 and exhaustgas outflow passages 61 are alternately arranged via thin partitionwalls 64. In other words, the exhaust gas inflow passages 60 and exhaustgas outflow passages 61 are arranged so that each exhaust gas inflowpassage 60 is surrounded by four exhaust gas outflow passages 61 andeach exhaust gas outflow passage 61 is surrounded by four exhaust gasinflow passages 60.

The filter 12 a is for example formed from a porous material such ascordierite. Therefore, the exhaust gas flowing into the exhaust gasinflow passage 60, as shown by the arrows in FIG. 2(B), passes throughthe surrounding partition walls 64 and flows out into the adjoiningexhaust gas outflow passages 61. When carrying the NOx catalyst 12 onthe filter 12 a in this way, the peripheral walls of the exhaust gasinflow passages 60 and exhaust gas outflow passages 61, that is, the twoside surfaces of the partition walls 64 and the inside walls of the fineholes in the partition walls 64, carry, for example, a catalyst carriercomprised of alumina. FIG. 3 schematically shows a cross-section of thesurface part of this catalyst carrier 45. As shown in FIG. 3, on thesurface of the catalyst carrier 45, a precious metal catalyst 46 iscarried diffused in it. Further, on the surface of the catalyst carrier45, a layer of an NOx adsorbent 47 is formed.

Further, in the embodiment of the present invention, as the preciousmetal catalyst 46, platinum (Pt) is used. As the ingredient forming theNOx adsorbent 47, for example, at least one ingredient selected frompotassium (K), sodium (Na), cesium (Cs), or another such alkali metal,barium (Ba), calcium (Ca), or another such alkali earth, and lanthanum(La), yttrium (Y), or another such rare earth is used.

If the ratio of the air and fuel (hydrocarbons) supplied inside theengine intake passage, combustion chambers 2, and exhaust passageupstream of the NOx catalyst 12 is referred to as the “air-fuel ratio ofthe exhaust gas”, the NOx adsorbent 47 absorbs the NOx when the air-fuelratio of the exhaust gas is leaner than even the stoichiometric air-fuelratio and releases the absorbed NOx when the oxygen concentration in theexhaust gas falls in an “NO_(x) absorption/release action”.

That is, explaining the case of using barium (Ba) as the ingredientforming the NOx adsorbent 47 as an example, when the air-fuel ratio ofthe exhaust gas is lean, that is, when the oxygen concentration in theexhaust gas is high, the NO contained in the exhaust gas, as shown inFIG. 3, is oxidized on the platinum 46 and becomes NO₂, next this isabsorbed in the NOx adsorbent 47 and, while bonding with the bariumoxide (BaO), diffuses in the form of nitric acid ions (NO₃ ⁻) inside theNOx adsorbent 47. In this way, the NOx is absorbed inside the NOxadsorbent 47. So long as the oxygen concentration in the exhaust gas ishigh, NO₂ is produced on the surface of the platinum 46. So long as theNOx adsorption ability of the NOx adsorbent 47 is not saturated, the NO₂is absorbed in the NOx adsorbent 47 and nitric acid ions (NO₃ ⁻) areproduced.

As opposed to this, if supplying hydrocarbons from the HC feed valve 14so as to make the air-fuel ratio of the exhaust gas the stoichiometricair-fuel ratio or richer than that, the oxygen concentration in theexhaust gas falls, so the reaction proceeds in the opposite direction(NO₃ ⁻→NO₂) and therefore the nitric acid ions (NO₃ ⁻) in the NOxadsorbent 47 are released in the form of NO₂ from the NOx adsorbent 47.Next, the released NOx is reduced by the unburned HC and CO contained inthe exhaust gas.

In this way, when the air-fuel ratio of the exhaust gas is lean, thatis, when combustion is performed under a lean air-fuel ratio, the NOx inthe exhaust gas is absorbed in the NOx adsorbent 47. However, whencombustion continues under a lean air-fuel ratio, during that time theNOx adsorption ability of the NOx adsorbent 47 ends up becomingsaturated and therefore the NOx adsorbent 47 ends up no longer beingable to absorb the NOx. Therefore, in the embodiment according to thepresent invention, before the adsorption ability of the NOx adsorbent 47becomes saturated, HC is supplied from the HC feed valve 14 so as totemporarily make the air-fuel ratio of the exhaust gas rich and therebymake the NOx be released from the NOx adsorbent 47.

However, exhaust gas contains SO_(x) (sulfur oxides), that is, SO₂. Ifthis SO₂ flows into the NOx catalyst 12, this SO₂ is oxidized at theplatinum 46 and becomes SO₃. Next, this SO₃ is adsorbed in the NOxadsorbent 47 and, while bonding with the barium oxide (BaO), diffuses inthe NOx adsorbent 47 in the form of sulfuric acid ions (SO₄ ²⁻) toproduce stable sulfate (BaSO₄). However, the NOx adsorbent 47 has astrong basicity, so this sulfate (BaSO₄) is stable and hard to breakdown. With just making the air-fuel ratio of the exhaust gas rich, thesulfate (BaSO₄) will not break down and will remain as it is. Therefore,in the NOx adsorbent 47, as time elapses, the sulfate (BaSO₄) increases.Therefore, along with the elapse of time, the NOx amount which can beabsorbed by the NOx adsorbent 47 falls.

However, in this case, as explained at the start, if making the air-fuelratio of the exhaust gas flowing into the NOx catalyst 11 rich in thestate of raising the temperature of the NOx catalyst 11 to the SO_(x)release temperature of 600° C. or more, the NOx adsorbent 47 is made torelease the SO_(x). However, in this case, the NOx adsorbent 47 onlyreleases a little SO_(x) at a time. Therefore, to make the NOx adsorbent47 release all of the absorbed SO_(x), the air-fuel ratio of the exhaustgas must be made rich over a long time and therefore there is theproblem that a large amount of fuel or reducing agent becomes necessary.Further, the SO_(x) released from the SO_(x) adsorbent 47 is exhaustedinto the atmosphere. This is also not preferable.

Therefore, in the embodiment according to the present invention, anSO_(x) trap 11 is arranged upstream of the NOx catalyst 12. This SO_(x)trap 11 is used to trap the SO_(x) contained in the exhaust gas andthereby prevent SO_(x) from being sent into the NOx catalyst 12. Next,this SO_(x) trap 11 will be explained.

This SO_(x) trap 11 is comprised of for example a honeycomb structuremonolithic catalyst and has a large number of exhaust gas circulationholes extending straight in the axial direction of the SO_(x) trap 11.When forming the SO_(x) trap 11 from a honeycomb structure monolithiccatalyst in this way, the inner circumferential walls of the exhaust gascirculation holes carry a catalyst carrier comprised of for examplealumina. FIG. 4 schematically shows the cross-section of the surfacepart of the catalyst carrier 50. As shown in FIG. 4, on the surface ofthe catalyst carrier 50, a coat layer 51 is formed and carries theprecious metal catalyst 52 diffused on its surface.

In the embodiment according to the present invention, as the preciousmetal catalyst 52, platinum (Pt) is used. As the ingredient forming thecoat layer 51, for example at least one element selected from potassium(K), sodium (Na), cesium (Cs), or another such alkali metal, barium(Ba), calcium (Ca), or another such alkali earth, and lanthanum (La),yttrium (Y), or another such rare earth is used. That is, the coat layer51 of the SOx trap 11 exhibits a strong basicity.

Further, the SOx contained in the exhaust gas, mainly SO₂, as shown inFIG. 4, is oxidized on the platinum 52, then is trapped in the coatlayer 51. That is, the SO₂ diffuses in the coat layer 51 in the form ofsulfuric acid ions (SO₄ ²⁻) and forms a sulfate. Note that in theabove-mentioned way, the coat layer 51 exhibits a strong basicity.Therefore, as shown in FIG. 4, part of the SO₂ contained in the exhaustgas is directly trapped in the coat layer 51.

Further, exhaust gas also contains particulate matter. The particulatematter contained in exhaust gas is trapped on the filter 12 a carryingthe NOx catalyst 12 and successively oxidized. However, if the amount ofthe trapped particulate matter becomes greater than the amount of theparticulate matter oxidized, the particulate matter gradually depositson the filter 12 a. In this case, if the amount of buildup of theparticulate matter increases, a drop in the engine output ends up beinginvited. Therefore, when the amount of buildup of the particulate matterincreases, the builtup particulate matter must be removed. In this case,if raising the temperature of the filter 12 a to 600° C. or so under anexcess of air, the builtup particulate matter is oxidized and removed.

Thus, in an embodiment of the present invention, when the amount ofparticulate matter built up on the filter 12 a exceeds the allowableamount, the temperature of the filter 12 a is raised under a leanair-fuel ratio of the exhaust gas and thereby the builtup particulatematter is oxidized and removed. Specifically speaking, in an embodimentof the present invention, when a pressure difference before and afterthe filter 12 a detected by a differential pressure sensor 23 exceeds anallowable value, it is judged that the amount of the builtup particulatematter has exceeded the allowable amount. At this time, temperatureelevation control is performed for raising the temperature of the filter12 a while keeping the air-fuel ratio of the exhaust gas flowing intothe filter 12 a lean.

In this regard, the SOx trapping action of the above-mentioned SOx trap11 is performed when the air-fuel ratio of the exhaust gas flowing intoit is an air-fuel ratio leaner than the stoichiometric air-fuel ratioand the temperature of the SOx trap 11 is higher than a certain constanttemperature (hereinafter referred to as the “activation temperature”).On the other hand, the SOx trap 11 ends up releasing the trapped SOxwhen the air-fuel ratio of the exhaust gas flowing into it becomes thestoichiometric air-fuel ratio or richer and its temperature becomeshigher than a certain constant temperature higher than the activationtemperature (hereinafter referred to as the “SOx release temperature”).Therefore, to prevent the SOx trap 11 from releasing SOx, it isnecessary to at least prevent the air-fuel ratio of the exhaust gasflowing into the SOx trap 11 from becoming the stoichiometric air-fuelratio or richer and to prevent the temperature of the SOx trap 11 frombecoming higher than even the SOx release temperature.

In this regard, even if the temperature of the SOx trap 11 as a wholebecomes lower than the SOx release temperature, sometimes thetemperature will locally become higher than even the SOx releasetemperature. At this time, the amount of SOx being trapped in the SOxtrap 11 (hereinafter referred to as “SOx trap amount”) becomesrelatively large and exhaust gas of the stoichiometric air-fuel ratio ora richer air-fuel ratio flows into the SOx trap 11, so there is apossibility of part of the SOx trap where the temperature locallybecomes higher than the SOx release temperature releasing SOx. Further,even if the air-fuel ratio of the exhaust gas flowing into the SOx trap11 becomes lean overall, sometimes it locally becomes rich. At thistime, if the SOx trap amount of the SOx trap 11 becomes relatively largeand the temperature of the SOx trap 11 becomes higher than the SOxrelease temperature, there is a possibility that part of the SOx trap 11will release SOx. That is, to reliably prevent SOx from being releasedfrom the SOx trap 11, when the SOx trap amount of the SOx trap 11becomes relatively large and the air-fuel ratio of the exhaust gasflowing into the SOx trap 11 becomes the stoichiometric air-fuel ratioor richer or when it is estimated it will become the stoichiometricair-fuel ratio or richer, it is necessary to prevent the temperature ofthe SOx trap 11 from becoming higher than the SOx release temperatureeven locally. In the same way, to reliably prevent the SOx trap 11 fromreleasing SOx, when the SOx trap amount of the SOx trap 11 becomesrelatively large and the temperature of the SOx trap 11 becomes higherthan the SOx release temperature or is estimated as becoming higher, itis necessary to prevent the air-fuel ratio of the exhaust gas flowinginto the SOx trap 11 from becoming the stoichiometric air-fuel ratio orricher even locally.

Here, in the above-mentioned way, when trying to make the NOx absorbent47 release NOx, HC is fed from the HC feed valve 14 into the exhaust gasto make the air-fuel ratio of the exhaust gas flowing into the NOxcatalyst 12 the stoichiometric air-fuel ratio or richer. Therefore, atthis time, the air-fuel ratio of the exhaust gas flowing into the SOxtrap 11 also becomes the stoichiometric air-fuel ratio or richer.Therefore, at this time, if the SOx trap amount of the SOx trap 11 isrelatively large, to reliably prevent the SOx trap 11 from releasingSOx, it is necessary to prevent the temperature of the SOx trap 11 frombecoming higher than the SOx release temperature even locally.

Thus, in an embodiment of the present invention, as the NOx releasecontrol for making the NOx absorbent 47 release NOx, when the SOx trapamount of the SOx trap 11 is smaller than a predetermined amount(hereinafter referred to as “the predetermined amount”), just NOxrelease control for making the NOx absorbent 47 release NOx (hereinafterreferred to as “ordinary NOx release control”) is executed, while whenthe SOx trap amount of the SOx trap 11 is larger than the predeterminedamount, SOx release suppression/NOx release control for keeping the SOxtrap 11 from releasing SOx while making the NOx absorbent 47 release NOxis executed.

Next, ordinary NOx release control and SOx release suppression/NOxrelease control employed as the NOx release control of an exhaustpurification system of the first embodiment will be explained. Note thatin the following description, the feed of HC from the HC feed valve 14into the exhaust gas will be referred to as “HC feed”, the amount of HCfed from the HC feed valve 14 into the exhaust gas per unit time in eachHC feed will be referred to as the “HC feed rate”, the time during whichthe HC is fed from the HC feed valve 14 into the exhaust gas in one HCfeed will be referred to as the “HC feed time”, the time interval atwhich each HC feed is performed will be referred to as the “HC feedinterval”, and the frequency by which HC feed is performed in oneordinary NOx release control or SOx release suppression/NOx releasecontrol will be referred to as the “HC feed frequency”.

The ordinary NOx release control of the first embodiment is performedwhen it is judged that the NOx absorbent 47 should release NOx and whenthe SOx trap amount of the SOx trap 11 is smaller than the predeterminedamount. In this ordinary NOx release control, as shown in FIG. 5(A), HCfeed with an HC feed rate of a predetermined HC feed rate (hereinafterreferred to as an “ordinary HC feed rate”) Qa and with an HC feed timeof a predetermined HC feed time (hereinafter referred to as an “ordinaryHC feed time”) Ta is performed at a predetermined HC feed interval(hereinafter referred to as the “ordinary HC feed interval”) Ia by apredetermined HC feed frequency (hereinafter referred to as an “ordinaryHC feed frequency”, in the example shown in FIG. 5(A), three times).

Note that in the ordinary NOx release control in the first embodiment,the HC feed rate in each HC feed, the HC feed time in each HC feed, andthe HC feed frequency are set so that the total amount of HC fed to theNOx catalyst 12 when all of the HC feed operations end becomes asufficient HC amount for making the NOx absorbent 47 release apredetermined amount of NOx (hereinafter referred to as the“predetermined HC amount”). Therefore, according to the ordinary NOxrelease control of the first embodiment, it is possible to make the NOxabsorbent 47 release a predetermined amount of NOx.

On the other hand, the SOx release suppression/NOx release control ofthe first embodiment is performed when it is judged that the NOxabsorbent 47 should release NOx and when the SOx trap amount of the SOxtrap 11 becomes greater than the predetermined amount. In this SOxrelease suppression/NOx release control, as shown in FIG. 5(B), HC feedwith an HC feed rate of the HC feed rate Qb smaller than the ordinary HCfeed rate Qa and with an HC feed time of a time Ta equal to the ordinaryHC feed time Ta is performed at an interval Ib shorter than the ordinaryHC feed interval Ia by a frequency larger than the ordinary HC feedfrequency. According to this, in one HC feed, the amount of HC fed fromthe HC feed valve 14 into the exhaust gas is small, so the HC fed fromthe HC feed valve 14 easily diffuses in the exhaust gas. For thisreason, formation of a region in the exhaust gas where the air-fuelratio locally very rich is suppressed, so the temperature of the SOxtrap 11 is kept from becoming higher than the locally SOx releasetemperature. Therefore, the SOx trap 11 is reliably kept from releasingSOx.

That is, if there is a region in the exhaust gas where the air-fuelratio locally becomes very rich, that is, a region in the exhaust gaswhere locally HC is included in a very large amount, the HC deposits toa partial region of the SOx trap 11 when the exhaust gas flows into theSOx trap 11. If the deposited HC is burned all at once in that partialregion of the SOx trap 11, there is a possibility that the temperatureof that partial region will become higher than the SOx releasetemperature. However, according to the SOx release suppression/NOxrelease control of the first embodiment, formation of a region in theexhaust gas where the air-fuel ratio locally becomes very rich issuppressed, so the temperature of the partial region of the SOx trap 11is kept from becoming higher than the SOx release temperature.Therefore, the temperature of the SOx trap 11 is kept from locallybecoming higher than the SOx release temperature and the SOx trap 11 isreliably kept from releasing SOx.

Note that in the SOx release suppression/NOx release control of thefirst embodiment, as shown in FIG. 5(C), HC feed with an HC feed rate ofan HC feed rate Qb smaller than the ordinary HC feed rate Qa and with anHC feed time of a time Tc longer than the ordinary HC feed time may beperformed at an interval Ic longer than the ordinary HC feed interval Iaby the same frequency as the ordinary HC feed frequency. According tothis, the HC feed rate in each HC feed operation is small, so the HC fedfrom the HC feed valve 14 easily diffuses in the exhaust gas. For thisreason, the temperature of the SOx trap 11 is kept from locally becominghigher than the SOx release temperature, so the SOx trap 11 is reliablykept from releasing SOx.

Note that in the SOx release suppression/NOx release control of thefirst embodiment, preferably the HC feed rate in each HC feed, the HCfeed time in each HC feed, and the HC feed frequency are set so that thetotal amount of HC fed to the NOx catalyst 12 when all of the HC feedoperations end becomes the predetermined HC amount. Thus, in the exampleshown in FIG. 5(B), the HC feed rate is made the HC feed rate Qb of halfof the ordinary HC feed rate Qa, the HC feed time is made the time Taequal to the ordinary HC feed time Ta, and the HC feed frequency is madea frequency double the ordinary HC feed frequency. Note that in theexample shown in FIG. 5(B), the HC feed interval is made an interval Ibof half of the ordinary HC feed interval Ia.

Further, in the example shown in FIG. 5(C), the HC feed rate is made theHC feed rate Qb of half of the ordinary HC feed rate Qa, the HC feedtime is made the time Tc of double the ordinary HC feed time Ta, and theHC feed frequency is made the same frequency as the ordinary HC feedfrequency.

Next, NOx release control of an exhaust purification system of thesecond embodiment will be explained with reference to FIG. 6. Note thatin FIGS. 6(A) to (C), the upper line shows the feed of HC from the HCfeed valve 14 into the exhaust gas, while the lower line shows theinjection of fuel from the fuel injector 3 in the latter half of theexpansion stroke or during an exhaust stroke of a specific cylinder.Further, in the following explanation, the injection of fuel from thefuel injector 2 in the latter half of the expansion stroke or during theexhaust stroke of a specific cylinder will be referred to as the “postfuel injection”, the amount of fuel injected from the fuel injector 2per unit time in each post fuel injection will be referred to as the“post fuel injection rate”, the time during which fuel is injected fromthe fuel injector 2 in one post fuel injection will be referred to asthe “post fuel injection time”, the time interval at which each postfuel injection is performed will be referred to as the “post fuelinjection interval”, and the frequency one post fuel injection isperformed will be referred to as the “post fuel injection frequency”.

In the NOx release control of the second embodiment, when it is judgedthat the NOx absorbent 47 should release NOx (that is, when the NOxrelease condition stands) and the SOx trap amount of the SOx trap 11 issmaller than the predetermined amount (that is, when the SOx releasesuppression condition does not stand), ordinary NOx release control isexecuted. In this ordinary NOx release control, as shown by the upperline of FIG. 6(A), an HC feed with an HC feed rate of an HC feed rate Qaequal to the ordinary HC feed rate Qa and with an HC feed time of a timeTa equal to the ordinary HC feed time Ta is performed at intervals Iaequal to the ordinary HC feed intervals Ia by the same frequency as theordinary frequency. Further, at this time, as shown by the lower line ofFIG. 6(A), none of the cylinders performs post fuel injection. Ofcourse, in the NOx release control of the second embodiment as well, theHC feed rate in each HC feed, the HC feed time in each HC feed, and theHC feed frequency are set so that the total amount of HC fed to the NOxcatalyst 12 when all of the HC feed operations end becomes thepredetermined HC amount.

On the other hand, in the NOx release control of the second embodiment,when the NOx release condition stands and the SOx trap amount of the SOxtrap 11 is larger than the predetermined amount (that is, when the SOxrelease suppression condition stands), SOx release suppression/NOxrelease control is executed. In this SOx release suppression/NOx releasecontrol, as shown by the upper line of FIG. 6(B), HC feed with an HCfeed rate of an HC feed rate Qb smaller than the ordinary HC feed rateQa and with an HC feed time of a time Ta equal to the ordinary feed timeis performed at an interval Ia equal to the ordinary HC feed interval Iaby the same frequency as the ordinary frequency and, as shown by thelower line of FIG. 6(B), post fuel injection with a post fuel injectionrate of a post fuel injection rate Qbp smaller than the ordinary HC feedrate Qa and with a post fuel injection time of a time Tap equal to theordinary HC feed time Ta is performed at an interval Iap equal to theordinary HC feed interval Ia by the same frequency as the ordinary HCfeed frequency. According to this, the amount of HC fed from the HC feedvalve 14 into the exhaust gas in one HC feed is small, so the HC fedfrom the HC feed valve 14 easily diffuses in the exhaust gas. For thisreason, the HC injected from the HC feed valve 14 is kept from causingthe temperature of the SOx trap 11 to locally become higher than the SOxrelease temperature. Furthermore, the fuel injected from the fuelinjector 3 at a specific cylinder in the latter half of the expansionstroke or during the exhaust stroke is modified by the heat in thecylinder and lightened. The thus lightened fuel passes through the SOxtrap 11 and is fed to the NOx catalyst 12, but this lightened fueleasily diffuses in the exhaust gas. For this reason, the fuel injectedfrom the fuel injector 3 at a specific cylinder in the latter half ofthe expansion stroke or during the exhaust stroke is kept from causingthe temperature of the SOx trap 11 from locally becoming higher than theSOx release temperature. Therefore, the SOx trap 11 is reliably keptfrom releasing SOx.

Note that in SOx release suppression/NOx release control of the secondembodiment, as shown in FIG. 6(C), it is also possible to use just postfuel injection to feed HC (fuel) to the NOx catalyst 12. That is, asshown by the lower line of FIG. 6(C), post fuel injection with a postfuel injection rate of a post fuel injection rate Qap equal to theordinary HC feed rate Qa and with a post fuel injection time of a timeTap equal to the ordinary HC feed time Ta may be performed at aninterval Iap equal to the ordinary HC feed interval Ia by the samefrequency as the ordinary HC feed frequency. Of course, at this time, asshown by the upper line of FIG. 6(C), HC is not fed from the HC feedvalve 14 into the exhaust gas. According to this, the fuel (HC) passingthrough the SOx trap 11 and fed to the NOx catalyst 12 is lightenedfuel, so easily diffuses in the exhaust gas. For this reason, thetemperature of the SOx trap 11 is kept from locally becoming higher thanthe SOx release temperature. Therefore, the SOx trap 11 is reliably keptfrom releasing SOx.

Note that in SOx release suppression/NOx release control of the secondembodiment, the HC feed rate in each HC feed, the HC feed time in eachHC feed, and the HC feed frequency and the post fuel injection rate ateach post fuel injection, the post fuel injection time at each post fuelinjection, and the post fuel injection frequency are preferably set sothat the total amount of HC (fuel) fed to the NOx catalyst 12 when allof the HC feed operations and all of the post fuel injections endbecomes the predetermined HC (fuel) amount. Thus, in the example shownin FIG. 6(B), the HC feed rate is made an HC feed rate Qb of half of theordinary HC feed rate Qa, the HC feed time is made a time Ta equal tothe ordinary HC feed time Ta, the HC feed frequency is made a frequencyequal to the ordinary HC feed frequency, the post fuel injection rate ismade a post fuel injection rate Qbp of half of the ordinary HC feed rateQa, the post injection time is made a time Tap equal to the ordinary HCfeed time Ta, and the post fuel injection frequency is made a frequencyequal to the ordinary HC feed frequency. Note that in the example shownin FIG. 6(B), the HC feed interval and the post fuel injection intervalare made intervals Ia, Iap equal to the ordinary HC feed interval Ia.

Further, in the example shown in FIG. 6(C), the post fuel injection rateis made a post fuel injection rate Qap equal to the ordinary HC feedrate Qa, the post fuel injection time is made a time Tap equal to theordinary HC feed time Ta, and the post fuel injection frequency is madea frequency equal to the ordinary HC feed frequency. Note that in theexample shown in FIG. 6(C), the post fuel injection interval is made aninterval Iap equal to the ordinary HC feed interval Ia.

Note that in the example shown in FIG. 6, the post fuel injection isshown executed at the same timing as the HC feed, but the post fuelinjection timing is controlled based on the crank angle of the internalcombustion engine, so strictly speaking, in most cases, the post fuelinjection timing will not become the same timing as the HC feed timing,but will deviate from it somewhat. Further, in the example shown in FIG.6, the post fuel injection interval was explained as equal to theordinary HC feed interval, but for the same reason, strictly speaking,in most cases, the post fuel injection interval will not become equal tothe ordinary HC feed interval, but will deviate from it somewhat.

Note that when performing post fuel injection to feed fuel to the NOxcatalyst 12, the HC fed to the NOx catalyst when performing the postfuel injection in the latter half of the expansion stroke has a higherdiffusion ability in the exhaust gas compared with HC fed to the NOxcatalyst 12 when performing the post fuel injection during the exhauststroke. Thus, in the above-mentioned embodiment, as NOx release control,only post fuel injection is employed as the method of feeding HC to theNOx catalyst 12. In ordinary NOx release control, post fuel injection isperformed during the exhaust stroke to feed HC to the NOx catalyst 12.On the other hand, in SOx release suppression/NOx release control, it isalso possible to feed HC to the NOx catalyst 12 by performing post fuelinjection in the latter half of the expansion stroke. This also enablesthe SOx trap 11 to be reliably kept from releasing SOx.

Next, NOx release control of an exhaust purification system of a thirdembodiment will be explained. In the NOx release control of the thirdembodiment, when the NOx release condition stands and the SOx releasesuppression condition does not stand, control the same as the ordinaryNOx release control of the first embodiment is executed.

On the other hand, in NOx release control of the third embodiment, whenthe NOx release condition stands and the SOx release suppressioncondition stands, SOx release suppression/NOx release control isexecuted. In this SOx release suppression/NOx release control, in thesame way as the ordinary NOx release control of the above-mentionedfirst embodiment, the ordinary HC feed rate, ordinary HC feed time, and,ordinary HC feed interval are used for performing each HC feed by anordinary HC feed frequency, but HC lightened by fractional distillationis prepared in advance and part of the HC fed from the HC feed valve 14into the exhaust gas in each HC feed is made this lightened HC. Asexplained above, lightened HC easily diffuses in the exhaust gas. Forthis reason, according to the SOx release suppression/NOx releasecontrol of the third embodiment, the temperature of the SOx trap 11 iskept from locally becoming higher than the SOx release temperature.Therefore, the SOx trap 11 is reliably kept from releasing SOx.

Next, NOx release control of an exhaust purification system of a fourthembodiment will be explained. In the NOx release control of the fourthembodiment, when the NOx release condition stands and the SOx releasesuppression condition does not stand, control the same as the ordinaryNOx release control of the first embodiment is executed.

On the other hand, in the NOx release control of the fourth embodiment,when the NOx release condition stands and the SOx release suppressioncondition stands, SOx release suppression/NOx release control isexecuted. In this SOx release suppression/NOx release control, the HCfeed rate in each HC feed, the HC feed time in each HC feed, and the HCfeed interval are controlled so that the temperature of the NOx catalyst12 is kept lower than the temperature of the NOx catalyst 12corresponding to the temperature of the SOx trap 11 at which the HC inthe exhaust gas will end up burning all at once in the SOx trap 11(hereinafter referred to as the “maximum NOx catalyst temperature”).That is, if the temperature of the NOx catalyst 12 is higher than themaximum NOx catalyst temperature, the temperature of the SOx trap 11becomes higher than the temperature at which the HC flowing into it endsup being made to burn all at once. In this case, when the HC fed fromthe HC feed valve 14 passes through the SOx trap 11, there is apossibility that it will burn all at once, the temperature of the SOxtrap 11 will locally become higher than the SOx release temperature, andthe SOx trap 11 will release SOx. On the other hand, according to SOxrelease suppression/NOx release control of the fourth embodiment, thetemperature of the NOx catalyst 12 is kept lower than the maximum NOxcatalyst temperature, so the HC flowing into the SOx trap 11 is keptfrom burning all at once. For this reason, the temperature of the SOxtrap 11 is kept from locally becoming higher than the SOx releasetemperature. Therefore, the SOx trap 11 is reliably kept from releasingSOx.

Note that in the SOx release suppression/NOx release control of thefourth embodiment, the HC feed rate in each HC feed, the HC feed time ineach HC feed, and the HC feed frequency are preferably set so that thetotal amount of HC fed into the NOx catalyst 12 when all of the HC feedoperations end becomes the predetermined amount.

Next, NOx release control of an exhaust purification system of a fifthembodiment will be explained. In the NOx release control of the fifthembodiment, when the NOx release condition stands and the SOx releasesuppression condition does not stand, control the same as the ordinaryNOx release control of the first embodiment is executed.

On the other hand, in the NOx release control of the fifth embodiment,when the NOx release condition stands and the SOx release suppressioncondition stands, SOx release suppression/NOx release control isexecuted. In this SOx release suppression/NOx release control, the HCfeed rate in each HC feed, the HC feed time in each HC feed, and the HCfeed interval are controlled so that the amplitude of the rise and fallof the temperature of the NOx catalyst 12 (hereinafter referred to as“temperature amplitude”) is kept smaller than the temperature amplitudeof the NOx catalyst 12 allowed in ordinary NOx release control. That is,in SOx release suppression/NOx release control, HC feed isintermittently performed, so the NOx catalyst 12 is intermittently fedHC. Further, when HC flows into the NOx catalyst 12, the heat ofreaction of the HC in the NOx catalyst 12 causes the temperature of theNOx catalyst 12 to rise, then fall. Here, the temperature amplitude ofthe NOx catalyst 12 being large means the amplitude of the rise or fallof the temperature of the SOx trap 11 is also large. Further, in thiscase, the temperature of the SOx trap 11 may at least locally becomehigher than the SOx release temperature. Here, if the SOx trap amount ofthe SOx trap 11 is greater than the predetermined amount, the SOx trap11 may release SOx. Thus, in the SOx release suppression/NOx releasecontrol of the fifth embodiment, the HC feed rate in each HC feed, theHC feed time in each HC feed, and the HC feed interval are controlled sothat the temperature amplitude of the NOx catalyst 12 is kept smallerthan the temperature amplitude of the NOx catalyst 12 allowed inordinary NOx release control. According to this, the temperature of theSOx trap 11 is kept from locally becoming higher than the SOx releasetemperature. Therefore, the SOx trap 11 is reliably kept from releasingSOx.

Note that in the SOx release suppression/NOx release control of thefifth embodiment, the HC feed rate in each HC feed, the HC feed time ineach HC feed, and the HC feed frequency are preferably set so that thetotal amount of HC fed into the NOx catalyst 12 when all of the HC feedoperations end becomes the predetermined amount.

Next, the NOx release control of the exhaust purification system of asixth embodiment will be explained. In the NOx release control of thesixth embodiment, when the NOx release condition stands and the SOxrelease suppression condition does not stand, control the same as theordinary NOx release control of the above-mentioned first embodiment isexecuted.

On the other hand, in the NOx release control of the sixth embodiment,when the NOx release condition stands and the SOx release suppressioncondition stands, the SOx release suppression/NOx release control isexecuted. In this SOx release suppression/NOx release control, the HCfeed rate in each HC feed, the HC feed time in each HC feed, and the HCfeed interval are controlled so that the rich degree of the air-fuelratio of the exhaust gas fed into the NOx catalyst 12 is kept smallerthan the target rich degree in ordinary NOx release control. That is,when the rich degree of the air-fuel ratio of the exhaust gas flowinginto the NOx catalyst 12 is large, the rich degree of the air-fuel ratioof the exhaust gas flowing into the SOx trap 11 also becomes large.Further, in this case, a region in the exhaust gas flowing into the SOxtrap 11 where the air-fuel ratio locally becomes very rich may beformed. However, according to the SOx release suppression/NOx releasecontrol of the sixth embodiment, formation of a region in the exhaustgas flowing into the SOx trap 11 where the air-fuel ratio locallybecomes very rich is suppressed. For this reason, the temperature of theSOx trap 11 is kept from locally becoming higher than the SOx releasetemperature. Therefore, the SOx trap 11 is reliably kept from releasingSOx.

Note that in the SOx release suppression/NOx release control of thesixth embodiment, the rich degree of the air-fuel ratio of the exhaustgas fed into the NOx catalyst 12 is for example estimated from theoutput of an air-fuel ratio sensor provided in the exhaust pipedownstream of the NOx catalyst 12.

Further, as explained above, the HC flowing into the SOx trap 11deposits at a partial region of the SOx trap 11. Here, if thetemperature of the region of the SOx trap 11 where the HC deposits islow, the deposited HC will not burn but will remain deposited there.Here, if the temperature of the region of the SOx trap 11 where HC hasdeposited rises to the combustion temperature of HC, the deposited HCmay burn all at once. That is, the lower the temperature of the SOx trap11, the more possible it is that the HC deposited on the SOx trap 11will burn all at once. Thus, in the SOx release suppression/NOx releasecontrol of the above-mentioned sixth embodiment, when the rich degree ofthe air-fuel ratio of the exhaust gas fed to the NOx catalyst 12 is keptsmaller than the target rich degree in ordinary NOx release control, thelower the temperature of the SOx trap 11, the smaller the rich degree ofthe air-fuel ratio of the exhaust gas fed to the NOx catalyst 12 may bekept.

FIG. 7 shows an example of the routine for executing the NOx releasecontrol of an embodiment of the present invention. In the routine ofFIG. 7, first, at step 10, it is judged if the NOx amount ΣNOX absorbedin the NOx absorbent 47 is greater than an allowable value α (ΣNOX>α)(that is, whether the NOx release condition stands). Here, when it isjudged that ΣNOX≦α, the routine is ended as is. On the other hand, whenit is judged that ΣNOX>α, the routine proceeds to step 11 where it isjudged if the SOx trap amount ΣSOX of the SOx trap 11 is greater than apredetermined amount β (ΣSOX>β) (that is, whether the SOx releasesuppression condition stands).

When it is judged at step 11 that ΣSOX>β, the routine proceeds to step12 where one of the SOx release suppression/NOx release control of theabove-mentioned first embodiment to sixth embodiment is executed. On theother hand, when it is judged at step 11 that ΣSOX≦β, the routineproceeds to step 13 where one of the ordinary NOx release control of theabove-mentioned first embodiment to sixth embodiment is executed.

In this regard, when, as explained above, the amount of particulatematter built up on the filter 12 a exceeds the allowable value (that is,the PM removal condition stands), control is executed for keeping theair-fuel ratio of the exhaust gas flowing into the filter 12 a leanwhile raising the temperature of the filter 12 a to a temperature of atleast the temperature where the particulate matter burns (hereinafterreferred to as “PM combustion temperature”) to burn off the particulatematter deposited on the filter 12 a (hereinafter referred to as “PMremoval control”). In this PM removal control, to keep the air-fuelratio of the exhaust gas flowing into the filter 12 a lean while raisingthe temperature of the filter 12 a, HC is fed from the HC feed valve 14into the exhaust gas in a range where the air-fuel ratio of the exhaustgas flowing into the filter 12 a is kept lean. That is, if the HC feedvalve 14 feeds HC into the exhaust gas, HC is fed to the filter 12 a. Atthis time, if the air-fuel ratio of the exhaust gas flowing into thefilter 12 a is kept lean, the HC burns on the filter 12 a. The heat ofcombustion generated at that time causes the temperature of the filter12 a to rise. In this way, basically, when PM removal control isexecuted, even if the HC feed valve 14 feeds HC into the exhaust gas,the air-fuel ratio of the exhaust gas flowing into the filter 12 a iskept lean, so the air-fuel ratio of the exhaust gas flowing into the SOxtrap 11 is also kept lean. Therefore, at this time, basically, the SOxtrap 11 will not release SOx.

In this regard, even if the air-fuel ratio of the exhaust gas flowinginto the SOx trap 11 was kept lean during the execution of the PMremoval control, if the SOx trap amount of the SOx trap 11 becomesrelatively large, if there are regions in the exhaust gas flowing intothe SOx trap 11 where the air-fuel ratio is locally rich and there areparts of the SOx trap 11 where the temperature is locally higher thanthe SOx release temperature, part of the SOx trap 11 may release SOx.Further, in PM removal control, HC is fed from the HC feed valve 14 soas to raise the temperature of the filter 12 a to a temperature of atleast the relatively high temperature PM combustion temperature, butpart of the HC fed from the HC feed valve 14 burns at the SOx trap 11.Therefore, during execution of PM removal control, the temperature ofthe SOx trap 11 also becomes a relatively high temperature. For thisreason, during execution of PM removal control, the temperature of theSOx trap 11 can be said to easily locally become higher than the SOxrelease temperature. Whichever the case, to reliably keep the SOx trap11 from releasing SOx during execution of PM removal control, when theSOx trap amount of the SOx trap 11 is relatively large, it is necessaryto suppress the formation of a region in the exhaust gas flowing intothe SOx trap 11 where the air-fuel ratio locally forms a rich region orto keep the temperature of the SOx trap 11 from locally becoming higherthan the SOx release temperature.

Thus, in an embodiment of the present invention, as the PM removalcontrol for removing the particulate matter built up on the filter 12 a,when the SOx trap amount of the SOx trap 11 is smaller than thepredetermined amount, just PM removal control for burning off theparticulate matter built up on the filter 12 a (hereinafter referred toas “ordinary PM removal control”) is executed. When the SOx trap amountof the SOx trap 11 is larger than the predetermined amount, PM removalcontrol for keeping the SOx trap 11 from releasing SOx while burning offthe particulate matter built up on the filter 12 a (hereinafter referredto as “SOx release suppression/PM removal control”) is executed.

Next, the PM removal control of an exhaust purification system of aseventh embodiment will be explained. In the PM removal control of theseventh embodiment, when the amount of the particulate matter built upon the filter 12 a has exceeded an allowable amount (that is, when thePM removal condition stands) and the SOx trap amount of the SOx trap 11is smaller than the predetermined amount (that is, when the SOx releasesuppression condition does not stand), ordinary PM removal control isexecuted. In this ordinary PM removal control, as shown in FIG. 8(A), HCfeed with an HC feed rate of a predetermined HC feed rate (hereinafterreferred to as “ordinary HC feed rate”) Qd and with an HC feed time of apredetermined HC feed time (hereinafter referred to as “ordinary HC feedtime”) Td is performed at a predetermined HC feed interval (hereinafterreferred to as “ordinary HC feed interval”) Id by a predetermined HCfeed frequency (hereinafter referred to as “ordinary HC feed frequency”,in the example shown in FIG. 8(A), three times).

Note that in the ordinary PM removal control of the seventh embodiment,the HC feed rate in each HC feed, the HC feed time in each HC feed, andthe HC feed frequency are set so as to raise the temperature of thefilter 12 a to the PM combustion temperature and so that the totalamount of HC fed to the filter 12 a when all of the HC feed operationsend becomes an HC amount sufficient for burning off exactly apredetermined amount of the particulate matter built up on the filter 12a (hereinafter referred to as “the predetermined HC amount”). Therefore,according to the ordinary PM removal control of the seventh embodiment,it is possible to burn off exactly a predetermined amount of theparticulate matter built up on the filter 12 a.

On the other hand, in the PM removal control of the seventh embodiment,when the PM removal condition stands and the SOx trap amount of the SOxtrap 11 is larger than the predetermined amount (that is, when the SOxrelease suppression condition stands), SOx release suppression/PMremoval control is executed. In this SOx release suppression/PM removalcontrol, as shown in FIG. 8(B), HC feed with a HC feed rate of an HCfeed rate Qe smaller than the ordinary HC feed rate Qd and with an HCfeed time of a time Td equal to the ordinary HC feed time Td isperformed at an interval Ie shorter than the ordinary HC feed intervalId by a frequency greater than the ordinary HC feed frequency. Accordingto this, the amount of HC fed from the HC feed valve 14 into the exhaustgas in one HC feed is small, so the HC fed from the HC feed valve 14easily diffuses in the exhaust gas. For this reason, the formation inthe exhaust gas of a region where the air-fuel ratio becomes locallyrich is suppressed; so the release of SOx from the SOx trap 11 isreliably suppressed.

Note that in the SOx release suppression/NOx release control of theseventh embodiment, as shown in FIG. 8(C), an HC feed with an HC feedrate of an HC feed rate Qe smaller than the ordinary HC feed rate Qd andwith an HC feed time of a time Tf longer than the ordinary HC feed timemay also be performed at an interval If longer than the ordinary HC feedinterval Id by the same frequency as the ordinary HC feed frequency.According to this, the HC feed rate in each HC feed is small, so the HCfed from the HC feed valve 14 easily diffuses in the exhaust gas. Forthis reason, the formation in the exhaust gas of a region where theair-fuel ratio becomes locally rich is suppressed, so the release of SOxfrom the SOx trap 11 is reliably suppressed.

Note that in the SOx release suppression/PM removal control of theseventh embodiment, the HC feed rate in each HC feed, the HC feed timein each HC feed, the HC feed interval, and the HC feed frequency are setto at least enable the temperature of the filter 12 a to be raised tothe PM combustion temperature.

Further, in the SOx release suppression/PM removal control of theseventh embodiment as well, the HC feed rate in each HC feed, the HCfeed time in each HC feed, and the HC feed frequency are preferably setso that the total amount of HC fed to the filter 12 a when all of the HCfeed operations end becomes the predetermined amount. Thus, in theexample shown in FIG. 8(B), the HC feed rate is made the HC feed rate Qeof half of the ordinary HC feed rate Qd, the HC feed time is made a timeTd equal to the ordinary HC feed time Td, and the HC feed frequency ismade a frequency twice the ordinary HC feed frequency. Note that in theexample shown in FIG. 8(Bs), the HC feed interval is made an interval Ieof half of the ordinary HC feed intervals Id.

Further, in the example shown in FIG. 8(C), the HC feed rate is made anHC feed rate Qe of half of the ordinary HC feed rate Qd, the HC feedtime is made a time Tf of double the ordinary HC feed time Td, and theHC feed frequency is made a frequency equal to the ordinary HC feedfrequency so that the total amount of HC fed to the filter 12 a when allof the HC feed operations end becomes the predetermined amount. Notethat in the example shown in FIG. 8(C), the HC feed interval is made aninterval of about 1.5 times the ordinary HC feed interval.

Next, the PM removal control of the exhaust purification system of aneighth embodiment will be explained with reference to FIG. 9. Note thatin FIGS. 9(A) to (C), the upper line shows the feed of HC from the HCfeed valve 14 into the exhaust gas, while the lower line shows theinjection of fuel from the fuel injector 3 at a specific cylinder in thelatter half of the expansion stroke or during the exhaust stroke.

In the PM removal control of the eighth embodiment, when the PM removalcondition stands and the SOx release suppression condition does notstand, ordinary PM removal control is executed. In this ordinary PMremoval control, as shown by the upper line of FIG. 9(A), an HC feedwith a HC feed rate of an HC feed rate Qd equal to the ordinary HC feedrate Qd and with an HC feed time of a time Td equal to the ordinary HCfeed time Td is performed at an interval Id equal to the ordinary HCfeed interval Id by the same frequency as the ordinary frequency.Further, at this time, as shown by the lower line of FIG. 9(A), nocylinder is performing post fuel injection. Of course, in the ordinaryPM removal control of the eighth embodiment as well, the HC feed rate ineach HC feed, the HC feed time in each HC feed, and the HC feedfrequency are set so as to raise the temperature of the filter 12 a tothe PM combustion temperature and so that the total amount of HC fed tothe filter 12 a when all of the HC feed operations end becomes thepredetermined amount.

On the other hand, in the PM removal control of the eighth embodiment,when the PM removal condition stands and the SOx release suppressioncondition stands, SOx release suppression/PM removal control isexecuted. In this SOx release suppression/PM removal control, as shownby the upper line of FIG. 9(B), an HC feed with an HC feed rate of an HCfeed rate Qe smaller than the ordinary HC feed rate Qd and with an HCfeed time of a time Td equal to the ordinary feed time is performed atan interval Id equal to the ordinary HC feed interval Id by a frequencythe same as the ordinary frequency and, as shown by the lower line ofFIG. 9(B), post fuel injection with a post fuel injection rate of a postfuel injection rate Qep smaller than the ordinary HC feed rate Qd andwith a post fuel injection time of a time Tdp equal to the ordinary HCfeed time Td is performed at an interval Idp equal to the ordinary HCfeed interval Id by a frequency the same as the ordinary HC feedfrequency. According to this, the amount of HC fed into the exhaust gasfrom the HC feed valve 14 in one HC feed is small, so the HC fed fromthe HC feed valve 14 easily diffuses in the exhaust gas. For thisreason, the HC fed from the HC feed valve 14 is kept from causing theformation of a region in the exhaust gas where the air-fuel ratiolocally becomes very rich. Furthermore, the fuel injected from the fuelinjector 3 at a specific cylinder in the latter half of the expansionstroke or during the exhaust stroke is modified by the heat in thecylinder and lightened. Further, this lightened fuel easily diffuses inthe exhaust gas. For this reason, the fuel injected from the fuelinjector 3 at a specific cylinder in the latter half of the expansionstroke or during the exhaust stroke is kept from causing the formationof a region in the exhaust gas where the air-fuel ratio locally becomesvery rich. Therefore, the SOx trap 11 is reliably kept from releasingSOx.

Note that in the SOx release suppression/PM removal control of theeighth embodiment, as shown in FIG. 9(C), it is also possible to usejust the post fuel injection to feed HC (fuel) into the filter 12 a.That is, as shown by the lower line of FIG. 9(C), post fuel injectionwith a post fuel injection rate of a post fuel injection rate Qdp equalto the ordinary HC feed rate Qd and with a post fuel injection time of atime Tdp equal to the ordinary HC feed time Td may be performed at aninterval Idp equal to the ordinary HC feed interval Id by a frequencyequal to the ordinary HC feed frequency. Of course, at this time, asshown by the upper line of FIG. 9(C), the HC feed valve 14 does not feedHC into the exhaust gas. According to this, the fuel (HC) passingthrough the SOx trap 11 and fed to the filter 12 a is lightened fuel, soeasily diffuses in the exhaust gas. For this reason, formation of aregion in the exhaust gas where the air-fuel ratio locally becomes veryrich is suppressed, so the SOx trap 11 is reliably kept from releasingSOx.

Note that in the SOx release suppression/NOx release control of theeighth embodiment, the HC feed rate in each HC feed, the HC feed time ineach HC feed, the HC feed interval, and the HC feed frequency are set soas to at least enable the temperature of the filter 12 a to be raised tothe PM combustion temperature.

Further, in the SOx release suppression/PM removal control of the eighthembodiment as well, the HC feed rate in each HC feed, the HC feed timein each HC feed, and the HC feed frequency and the post fuel injectionrate in each post fuel injection, the post fuel injection time in eachpost fuel injection, and the post fuel injection frequency arepreferably set so that the total amount of HC (fuel) fed to the filter12 a when all of the HC feed operations and all of the post fuelinjections end becomes the predetermined HC (fuel). Thus, in the exampleshown in FIG. 9(B), the HC feed rate is made an HC feed rate Qe of halfof the ordinary HC feed rate Qd, the HC feed time is made a time Tdequal to the ordinary HC feed time Td, the HC feed frequency is made afrequency equal to the ordinary HC feed frequency, the post fuelinjection rate is made a post fuel injection rate Qdp of half of theordinary HC feed rate Qd, the post injection time is made a time Tdpequal to the ordinary HC feed time Td, and the post fuel injectionfrequency is made a frequency equal to the ordinary HC feed frequency.Note that in the example shown in FIG. 9(B), the HC feed interval andthe post fuel injection intervals are both made intervals Id, Idp equalto the ordinary HC feed interval Id.

Further, in the example shown in FIG. 9(C), the post fuel injection rateis made a post fuel injection rate Tdp equal to the ordinary HC feedrate Qd, the post fuel injection time is made a time Tdp equal to theordinary HC feed time Td, and the post fuel injection frequency is madea frequency equal to the ordinary HC feed frequency so that the totalamount of HC (fuel) fed to the filter 12 a when all of the HC feedoperations and all of the post fuel injections end becomes thepredetermined HC (fuel). Note that in the example shown in FIG. 9(C),the post fuel injection interval is made an interval Tdp equal to theordinary HC feed interval Id.

Note that in the example shown in FIG. 9, the post fuel injection isshown executed at the same timing as the HC feed, but the post fuelinjection timing is controlled based on the crank angle of the internalcombustion engine, so strictly speaking, in most cases, the post fuelinjection timing will not become the same timing as the HC feed timing,but will deviate from it somewhat. Further, in the example shown in FIG.9, the post fuel injection interval was explained as equal to theordinary HC feed interval, but for the same reason, strictly speaking,in most cases, the post fuel injection interval will not become equal tothe ordinary HC feed interval, but will deviate from it somewhat.

Next, PM removal control of an exhaust purification system of a ninthembodiment will be explained. In the PM removal control of the ninthembodiment, when the PM removal condition stands and the SOx releasesuppression condition does not stand, the same control as the ordinaryPM removal control of the seventh embodiment is executed.

On the other hand, in the PM removal control of the ninth embodiment,when the PM removal condition stands and the SOx release suppressioncondition stands, SOx release suppression/PM removal control isexecuted. In this SOx release suppression/PM removal control, in thesame way as the ordinary NOx release control of the seventh embodiment,the ordinary HC feed rate, ordinary HC feed time, and ordinary HC feedinterval are used for performing each HC feed by the ordinary HC feedfrequency, but HC lightened by fractional distillation is prepared inadvance and, in each HC feed, part of the HC fed from the HC feed valve14 into the exhaust gas is made this lightened HC. In theabove-mentioned way, the lightened HC easily diffuses in the exhaustgas. For this reason, the formation in the exhaust gas of a region wherethe air-fuel ratio becomes locally rich is suppressed, so the release ofSOx from the SOx trap 11 is reliably suppressed.

Next, the PM removal control of the exhaust purification system of the10th embodiment will be explained. In the PM removal control of the 10thembodiment, when the PM removal condition stands and the SOx releasesuppression condition does not stand, the same control as in theordinary PM removal control of the seventh embodiment is executed.

On the other hand, in the PM removal control of the 10th embodiment,when the PM removal condition stands and the SOx release suppressioncondition stands, the SOx release suppression/PM removal control isexecuted. In this SOx release suppression/PM removal control, the HCfeed rate in each HC feed, the HC feed time in each HC feed, and the HCfeed interval are controlled so that the temperature of the SOx trap 11is kept lower than the temperature where the HC in the exhaust gas endsup being burned all at once at the SOx trap 11. According to this, evenwhen a region where the air-fuel ratio becomes locally rich is formed inthe exhaust gas, the HC is kept from burning all at once at the SOx trap11. For this reason, the temperature of the SOx trap 11 is kept fromlocally becoming higher than the SOx release temperature, so the SOxtrap 11 is reliably kept from releasing SOx.

Note that in the SOx release suppression/PM removal control of the 10thembodiment, the HC feed rate in each HC feed, the HC feed time in eachHC feed, the HC feed interval, and the HC feed frequency are set so asto at least enable the temperature of the filter 12 a to be raised tothe PM combustion temperature.

Further, in the SOx release suppression/PM removal control of the 10thembodiment as well, the HC feed rate in each HC feed, the HC feed timein each HC feed, and the HC feed frequency are preferably set so thatthe total amount of HC fed to the filter 12 a becomes the predeterminedamount when all of the HC feed operations end.

Next, the PM removal control of an exhaust purification system of the11th embodiment will be explained. In the PM removal control of the 11thembodiment, when the PM removal condition stands and the SOx releasesuppression condition does not stand, control the same as the ordinaryPM removal control of the seventh embodiment is executed.

On the other hand, in the PM removal control of the 11th embodiment,when the PM removal condition stands and the SOx release suppressioncondition stands, SOx release suppression/PM removal control isexecuted. In this SOx release suppression/PM removal control, the HCfeed rate in each HC feed, the HC feed time in each HC feed, and the HCfeed interval are controlled so that the temperature of the filter 12 ais kept at a temperature as close as possible to the PM combustiontemperature. According to this, the HC feed rate at one HC feed is setsmall, the HC feed time in one HC feed is set short, or the HC feedinterval is set long. Therefore, the HC fed from the HC feed valve 14easily diffuses in the exhaust gas. For this reason, the formation inthe exhaust gas of a region where the air-fuel ratio becomes locallyrich is suppressed, so the release of SOx from the SOx trap 11 isreliably suppressed.

Note that in the SOx release suppression/PM removal control of the 11thembodiment, the HC feed rate in each HC feed, the HC feed time in eachHC feed, the HC feed interval, and the HC feed frequency are set so asto at least enable the temperature of the filter 12 a to be raised tothe PM combustion temperature.

Further, in the SOx release suppression/PM removal control of the 11thembodiment as well, the HC feed rate in each HC feed, the HC feed timein each HC feed, and the HC feed frequency are preferably set so thatthe total amount of HC fed to the filter 12 a becomes the predeterminedamount when all of the HC feed operations end. In this case, the timeduring which the SOx release suppression/PM removal control is executedbecomes longer than the time during which ordinary PM removal control isexecuted.

Next, PM removal control of an exhaust purification system of a 12thembodiment will be explained. In the PM removal control of the 12thembodiment, when the PM removal condition stands and the SOx releasesuppression condition does not stand, the same control as the ordinaryPM removal control of the seventh embodiment is executed.

On the other hand, in the PM removal control of the 12th embodiment,when the PM removal condition stands and the SOx release suppressioncondition stands, SOx release suppression/PM removal control isexecuted. In this SOx release suppression/PM removal control, the HCfeed rate in each HC feed, the HC feed time in each HC feed, and the HCfeed interval are controlled so that the width of the rise or fall ofthe temperature of the SOx trap 11 (hereinafter referred to as the“temperature amplitude”) is kept smaller than the temperature amplitudeof the SOx trap 11 allowed in ordinary PM removal control. According tothis, compared with during execution of ordinary PM removal control, theHC feed rate in each HC feed is set smaller, the HC feed time in each HCfeed is set shorter, or the HC feed interval is set longer. For thisreason, the HC fed from the HC feed valve 14 easily diffuses in theexhaust gas. Therefore, the formation in the exhaust gas of a regionwhere the air-fuel ratio becomes locally rich is suppressed, so therelease of SOx from the SOx trap 11 is reliably suppressed.

Note that in the SOx release suppression/PM removal control of the 12thembodiment, the HC feed rate in each HC feed, the HC feed time in eachHC feed, and the HC feed frequency are set to enable at least thetemperature of the filter 12 a to be raised to the PM combustiontemperature.

Further, in the SOx release suppression/PM removal control of the 12thembodiment as well, the HC feed rate in each HC feed, the HC feed timein each HC feed, and the HC feed frequency are preferably set so thatthe total amount of HC fed to the filter 12 a when all of the HC feedsend becomes a predetermined HC amount.

Next, the PM removal control of the exhaust purification system of the13th embodiment will be explained. In the PM removal control of the 13thembodiment, when the PM removal condition stands and the SOx releasesuppression condition does not stand, the same control as the ordinaryPM removal control of the seventh embodiment is executed.

On the other hand, in the PM removal control of the 13th embodiment,when the PM removal condition stands and the SOx release suppressioncondition stands, the SOx release suppression/PM removal control isexecuted. In this SOx release suppression/PM removal control, the HCfeed rate in each HC feed, the HC feed time in each HC feed, and the HCfeed interval are controlled so that the lean degree of the air-fuelratio of the exhaust gas fed to the filter 12 a is kept larger than thetarget lean degree in the ordinary PM removal control. That is, when thelean degree of the air-fuel ratio of the exhaust gas flowing into thefilter 12 a is small, the lean degree of air-fuel ratio of the exhaustgas flowing into the SOx trap 11 also becomes small. Further, in thiscase, a region in the exhaust gas flowing into the SOx trap 11 where theair-fuel ratio locally becomes rich may be formed. However, according tothe SOx release suppression/PM removal control of the 13th embodiment,the formation of a region in the exhaust gas flowing into the SOx trap11 where the air-fuel ratio becomes locally rich is suppressed, so therelease of SOx from the SOx trap 11 is reliably suppressed.

Note that in the SOx release suppression/PM removal control of the 13thembodiment, the HC feed rate in each HC feed, the HC feed time in eachHC feed, and the HC feed frequency are set to enable at least thetemperature of the filter 12 a to be raised to the PM combustiontemperature.

Further, in the SOx release suppression/PM removal control of the 13thembodiment as well, the HC feed rate in each HC feed, the HC feed timein each HC feed, and the HC feed frequency are preferably set so thatthe total amount of HC fed to the filter 12 a when all of the HC feedsend becomes a predetermined HC amount.

Further, in the SOx release suppression/PM removal control of the 13thembodiment, the lean degree of the air-fuel ratio of the exhaust gas fedto the filter 12 a is, for example, estimated from the output of theair-fuel ratio sensor attached to exhaust pipe downstream of the filter12 a.

Next, the PM removal control of an exhaust purification system of the14th embodiment will be explained. In the PM removal control of the 14thembodiment, when the PM removal condition stands and the SOx releasesuppression condition does not stand, control the same as the ordinaryPM removal control of the seventh embodiment is executed.

On the other hand, in the PM removal control of the 14th embodiment,when the PM removal condition stands and the SOx release suppressioncondition stands, SOx release suppression/PM removal control isexecuted. In this SOx release suppression/PM removal control, the HCfeed rate in each HC feed, the HC feed time in each HC feed, and the HCfeed interval are controlled so that the temperature elevation rate whenthe filter 12 a is raised in temperature is kept smaller than the targettemperature elevation rate in ordinary PM removal control. According tothis, the HC feed rate in one HC feed is set smaller, the HC feed timein one HC feed is set shorter, or the HC feed interval is set longer.For this reason, the HC fed from the HC feed valve 14 easily diffuses inthe exhaust gas. For this reason, the formation in the exhaust gas of aregion where the air-fuel ratio becomes locally rich is suppressed, sothe release of SOx from the SOx trap 11 is reliably suppressed.

Note that in the SOx release suppression/PM removal control of the 14thembodiment, the HC feed rate in each HC feed, the HC feed time in eachHC feed, the HC feed interval, and the HC feed frequency are set so asto at least enable the temperature of the filter 12 a to be raised tothe PM combustion temperature.

Further, in the SOx release suppression/PM removal control of the 14thembodiment as well, the HC feed rate in each HC feed, the HC feed timein each HC feed, and the HC feed frequency are preferably set so thatthe total amount of HC fed to the filter 12 a when all of the HC feedoperations end becomes the predetermined amount.

FIG. 10 shows an example of a routine for executing the PM removalcontrol of an embodiment of the present invention. In the routine ofFIG. 10, first, at step 20, it is judged if the amount ΣPM ofparticulate matter deposited on the filter 12 a is greater than anallowable value γ (ΣPM>γ) (that is, if the PM removal condition stands).Here, when it is judged that ΣPM≦γ, the routine is ended as is. On theother hand, when it is judged that ΣPM>γ, the routine proceeds to step21 where it is judged if the SOx trap amount ΣSOX of the SOx trap 11 isgreater than a predetermined amount β (ΣSOX>β) (that is, whether the SOxrelease suppression condition stands).

When it is judged at step 21 that ΣSOX>β, the routine proceeds to step22 where the SOx release suppression/PM removal control of one of theabove-mentioned seventh embodiment to the 14th embodiment is executed.On the other hand, when it is judged at step 21 that ΣSOX≦β, the routineproceeds to step 23 where the SOx release suppression/PM removal controlof one of the above-mentioned seventh embodiment to the 14th embodimentis executed.

However, when trying to make the NOx absorbent 47 release NOx and thetemperature of the SOx trap 11 becomes higher than the SOx releasetemperature, if feeding HC from the HC feed valve 14 into the exhaustgas so as to make the NOx absorbent 47 release NOx, the SOx trap 11 endsup releasing SOx. Thus, as NOx release control of an exhaustpurification system of the 15th embodiment, when trying to make the NOxabsorbent 47 release NOx (that is, when the NOx release conditionstands), it is possible to prohibit the feed of HC from the HC feedvalve 14 into the exhaust gas (that is, the execution of NOx releasecontrol in the above-mentioned embodiment) if the temperature of the SOxtrap 11 becomes higher than the SOx release temperature. According tothis, the SOx trap 11 is reliably kept from releasing SOx.

FIG. 11 shows an example of a routine executing NOx release control of a15th embodiment. In the routine of FIG. 11, first, at step 30, it isjudged if the NOx amount ΣNOX absorbed in the NOx absorbent 47 isgreater than an allowable value α (ΣNOX>α) (that is, whether the NOxrelease condition stands). Here, when it is judged that ΣNOX≦α, theroutine is ended as is. On the other hand, when it is judged thatΣNOX>α, the routine proceeds to step 31 where it is judged if thetemperature Tsox of the SOx trap 11 is the SOx release temperature Tthor more (Tsox≧Tth).

When it is judged at step 31 that Tsox≧Tth, the routine proceeds to step32 where execution of NOx release control is prohibited. That is, inthis case, the NOx release control is not executed. On the other hand,when it is judged at step 31 that Tsox<Tth, the routine proceeds to step33, where it is judged if the SOx trap amount ESOX of the SOx trap 11 isgreater than a predetermined amount β (ΣSOX>β) (that is, whether the SOxrelease suppression condition stands).

When it is judged at step 33 that ΣSOX>β, the routine proceeds to step34, where SOx release suppression/NOx release control of one of theabove-mentioned first embodiment to sixth embodiment is executed. On theother hand, when it is judged at step 33 that ΣSOX≦β, the routineproceeds to step 35 where one of the ordinary NOx release control of theabove-mentioned first embodiment to sixth embodiment is executed.

However, as the PM removal control of the exhaust purification system ofthe 16th embodiment, the following control may also be employed. Thatis, as explained above, during execution of PM removal control, thetemperature of the SOx trap 11 becomes relatively high, but here whenthe temperature of the SOx trap 11 is higher than the SOx releasetemperature, compared with when the temperature of the SOx trap 11 islower than the SOx release temperature, the formation in the exhaust gasof a region where the air-fuel ratio locally becomes rich should bereliably suppressed. Thus, in the PM removal control of the 16thembodiment, when the PM removal condition stands and the SOx releasesuppression condition does not stand, one of the SOx releasesuppression/PM removal control of the above-mentioned seventh embodimentto 14th embodiment is executed.

On the other hand, in the PM removal control of the 16th embodiment,when the PM removal condition stands and the SOx release suppressioncondition stands, it is judged that the temperature of the SOx trap 11is higher than the SOx release temperature. Here, when the temperatureof the SOx trap 11 is lower than the SOx release temperature, either ofthe SOx release suppression/PM removal control of the above-mentionedseventh embodiment to the 14th embodiment is executed. On the otherhand, when the temperature of the SOx trap 11 is higher than the SOxrelease temperature, similar control as the SOx release suppression/PMremoval control performed when the temperature of the SOx trap 11 islower than the SOx release temperature is executed, but the HC feed rateat this time is made smaller than the HC feed rate in the SOx releasesuppression/PM removal control performed when the temperature of the SOxtrap 11 is lower than the SOx release temperature. According to this,the amount of HC fed from the HC feed valve 14 in one HC feed is small,so the HC fed from the HC feed valve 14 easily diffuses in the exhaustgas. For this reason, the formation in the exhaust gas of a region wherethe air-fuel ratio becomes locally rich is suppressed, so the release ofSOx from the SOx trap 11 is suppressed.

Alternatively, in the above-mentioned PM removal control of the 16thembodiment, when the PM removal condition stands and the SOx releasesuppression condition stands, the HC feed rate in each HC feed, the HCfeed time in each HC feed, and the HC feed interval may be controlled sothat the width of the rise or fall of the temperature of the SOx trap 11(temperature amplitude) when the temperature of the SOx trap 11 ishigher than the SOx release temperature is kept smaller than thetemperature amplitude of the SOx trap 11 allowed in the SOx release/PMremoval control performed when the temperature of the SOx trap 11 islower than the SOx release temperature. According to this, compared withexecution of the SOx release suppression/PM removal control performedwhen the temperature of the SOx trap 11 is lower than the SOx releasetemperature, the HC feed rate in each HC feed is set smaller, the HCfeed time in each HC feed is set shorter, or the HC feed interval is setlonger. For this reason, the HC fed from the HC feed valve 14 easilydiffuses in the exhaust gas. Therefore, the formation in the exhaust gasof a region where the air-fuel ratio becomes locally rich is suppressed,so the release of SOx from the SOx trap 11 is reliably suppressed.

FIG. 12 shows an example of the routine executing the PM removal controlof the 16th embodiment. In the routine of FIG. 12, first, at step 40, itis judged if the amount ΣPM of the particulate matter deposited on thefilter 12 a is greater than an allowable value γ (ΣPM>γ) (that is,whether the PM removal condition stands). Here, when it is judged thatΣPM≦γ, the routine is ended as is. On the other hand, when it is judgedthat ΣPM>γ, the routine proceeds to step 41 where it is judged if theSOx trap amount ΣSOX of the SOx trap 11 is greater than a predeterminedamount γ (ΣSOX>β) (that is, whether the SOx release suppressioncondition stands).

When it is judged at step 41 that ΣSOX≦β, the routine proceeds to step45 where one of the ordinary PM removal control of the seventhembodiment to 14th embodiment is executed. On the other hand, when it isjudged at step 41 that ΣSOX>β, the routine proceeds to step 42 where itis judged if the temperature Tsox of the SOx trap 11 is the SOx releasetemperature Tth or more (Tsox≧Tth).

When it is judged at step 42 that Tsox<Tth, the routine proceeds to step44 where SOx release suppression/PM removal control II is executed. Inthis SOx release suppression/PM removal control II, one of the SOxrelease suppression/PM removal control of the seventh embodiment to the14th embodiment is executed. On the other hand, when it is judged atstep 42 that Tsox≧Tth, the routine proceeds to step 45 where SOx releasesuppression/PM removal control I is executed. In this SOx releasesuppression/PM removal control I, control similar to the SOx releasesuppression/PM removal control II of step 44 is executed, but here theHC feed rate is made smaller than the HC feed rate at the SOx releasesuppression/PM removal control II of step 44.

In this regard, it is also possible to employ the following control asNOx release control of an exhaust purification system of a 17thembodiment. That is, in the NOx release control of the 17th embodiment,when the lean degree of the air-fuel ratio of the exhaust gas exhaustedfrom each cylinder is larger than a predetermined lean degree(hereinafter referred to as “the predetermined lean degree”), one of theordinary NOx release control of the first embodiment to sixth embodimentis executed when the NOx release condition stands. On the other hand,when the lean degree of the air-fuel ratio of the exhaust gas exhaustedfrom each cylinder is smaller than the predetermined lean degree, one ofthe SOx release suppression/NOx release control of the first embodimentto sixth embodiment is executed when the NOx release condition stands.According to this, the SOx trap 11 is reliably kept from releasing SOx.

That is, when the lean degree of the air-fuel ratio of the exhaust gasexhausted from each cylinder is smaller than the predetermined leandegree, the air-fuel ratio of the exhaust gas becomes close to a richair-fuel ratio. At this time, if NOx release control is executed, thereis a high possibility that a region in the exhaust gas flowing into theSOx trap 11 where the air-fuel ratio locally becomes very rich will beformed. Therefore, there is a high possibility that the temperature ofthe SOx trap 11 will locally become higher than the SOx releasetemperature. Therefore, when executing NOx release control, if the leandegree of the air-fuel ratio of the exhaust gas exhausted from eachcylinder is smaller than the predetermined lean degree, to reliably keepthe SOx trap 11 from releasing SOx, formation of a region in the exhaustgas where the air-fuel ratio locally becomes very rich is suppressed.Therefore, it is necessary to keep the temperature of the SOx trap 11from locally becoming higher than the SOx release temperature. Thus, inthe NOx release control of the 17th embodiment, when the lean degree ofthe air-fuel ratio of the exhaust gas exhausted from each cylinder issmaller than the predetermined lean degree, one of the SOx releasesuppression/NOx release control of the first embodiment to sixthembodiment is executed.

In this regard, as the PM removal control of the exhaust purificationsystem of the 18th embodiment, the following control may be employed.That is, in the PM removal control of the 18th embodiment, when the leandegree of the air-fuel ratio of the exhaust gas exhausted from eachcylinder is larger than a predetermined lean degree (hereinafterreferred to as “the predetermined lean degree”), one of the ordinary PMremoval control of the seventh embodiment to 14th embodiment is executedwhen the PM removal condition stands. On the other hand, when the leandegree of the air-fuel ratio of the exhaust gas exhausted from eachcylinder is smaller than the predetermined lean degree, one of the SOxrelease suppression/PM removal control of the seventh embodiment to the14th embodiment is executed when the PM removal condition stands.According to this, the SOx trap 11 is reliably kept from releasing SOx.

That is, when the lean degree of the air-fuel ratio of the exhaust gasexhausted from each cylinder is smaller than the predetermined leandegree, the air-fuel ratio of the exhaust gas becomes close to a richair-fuel ratio. At this time, if PM removal control is executed, thereis a high possibility that a region in the exhaust gas flowing into theSOx trap 11 where the air-fuel ratio locally becomes very rich will beformed. Therefore, when executing PM removal control, if the lean degreeof the air-fuel ratio of the exhaust gas exhausted from each cylinder issmaller than the predetermined lean degree, to reliably keep the SOxtrap 11 from releasing SOx, formation of a region in the exhaust gaswhere the air-fuel ratio locally becomes very rich has to be suppressed.Therefore, in the PM removal control of the 18th embodiment, when thelean degree of the air-fuel ratio of the exhaust gas exhausted from eachcylinder is smaller than the predetermined lean degree, one of the SOxrelease suppression/NOx release control of the seven embodiment to 14thembodiment is executed.

Note that in the PM removal control of the 18th embodiment, it is alsopossible to prohibit execution of the PM removal control when the leandegree of the air-fuel ratio of the exhaust gas exhausted from eachcylinder is smaller than the predetermined lean degree. This alsoenables the SOx trap 11 to be reliably kept from releasing SOx.

Further, the NOx release control and PM removal control of theabove-mentioned embodiments can also be applied to the compressionignition type of internal combustion engine shown in FIG. 13. Theinternal combustion engine shown in FIG. 13 is similar to the internalcombustion engine shown in FIG. 1, but in the internal combustion engineshown in FIG. 13, instead of the NOx catalyst 12 carried on the filter12 a, a particulate filter 12 a for trapping particulate matter isarranged downstream of the SOx trap 11 and an NOx catalyst 12 isarranged downstream of the particulate filter 12 a. Further, in theinternal combustion engine shown in FIG. 13, when trying to make the NOxabsorbent of the NOx catalyst 12 release NOx, the NOx release control ofthe above-mentioned embodiments is adopted. Further, in the internalcombustion engine shown in FIG. 13, when trying to burn off theparticulate matter built up on the particulate filter 12 a, the PMremoval control of above-mentioned embodiments is employed.

Note that in the internal combustion engine shown in FIG. 13, theparticulate filter 12 a is provided with a temperature sensor 22 fordetecting the temperature of the particulate filter 12 a and adifferential pressure sensor 23 for detecting the differential pressurebefore and after the particulate filter 12 a. Further, the NOx catalyst12 is provided with a temperature sensor 24 for detecting thetemperature of the NOx catalyst 12.

Further, the NOx release control and PM removal control of theabove-mentioned embodiments may also be applied to the compressionignition type of internal combustion engine shown in FIG. 14. Theinternal combustion engine shown in FIG. 14 is similar to the internalcombustion engine shown in FIG. 1, but in the internal combustion engineshown in FIG. 14, downstream of the SOx trap 11, instead of the NOxcatalyst 12 carried on the filter 12 a, an NOx catalyst 12 is arrangedand, downstream of the NOx catalyst 12, just a particulate filter 12 afor trapping particulate matter is arranged. Further, in the internalcombustion engine shown in FIG. 14, when trying to make the NOxabsorbent of the NOx catalyst 12 release NOx, the NOx release control ofthe above-mentioned embodiments is employed. Further, in the internalcombustion engine shown in FIG. 14, when trying to burn off theparticulate matter built up on the particulate filter 12 a, the PMremoval control of the above-mentioned embodiments is employed.

Further, in the internal combustion engine shown in FIG. 1, as shown inFIG. 15, it is also possible to arrange upstream of the SOx trap 11 anoxidation catalyst 26 oxidizing the HC fed from the HC feed valve 14into the exhaust gas and provided with an oxidizing ability higher thanthe oxidizing ability of the SOx trap 11. In this case, the HC fed fromthe HC feed valve 14 into the exhaust gas is oxidized by the oxidationcatalyst 26, so formation of a region in the exhaust gas where theair-fuel ratio locally becomes rich is reliably suppressed.

Further, in the exhaust purification system of the above-mentionedembodiment, the HC feed valve 14 is provided with a heater for heatingthe HC feed valve 14. In ordinary NOx release control or ordinary PMremoval control, when the HC feed valve 14 feeds HC into the exhaustgas, the HC feed valve 14 is not heated by the heater, but in SOxrelease suppression/NOx release control or SOx release suppression/PMremoval control, when the HC feed valve 14 feeds HC into the exhaustgas, the HC feed valve 14 may be heated by the heater. According tothis, in SOx release suppression/NOx release control or SOx releasesuppression/PM removal control, HC fed from the HC feed valve 14 easilydiffuses in the exhaust gas, so SOx is kept from being released from theSOx trap 11.

Further, in the SOx release suppression/NOx release control or SOxrelease suppression/PM removal control of the above-mentionedembodiments, the pressure for feeding HC from the HC feed valve 14 intothe exhaust gas may be made higher than the pressure for feeding HC fromthe HC feed valve 14 into the exhaust gas in ordinary NOx releasecontrol or ordinary PM removal control. Due to this as well, in SOxrelease suppression/NOx release control or SOx release suppression/PMremoval control, the HC fed from the HC feed valve 14 becomes-able toeasily diffuse in the exhaust gas, so SOx is kept from being releasedfrom the SOx trap 11.

Further, in the exhaust purification system of the above-mentionedembodiment, as the HC feed valve 14, an HC feed valve provided with aplurality of feed ports for feeding HC and enabling the number of feedports feeding HC to be suitably changed is employed. In the SOx releasesuppression/NOx release control or SOx release suppression/PM removalcontrol, when the HC feed valve feeds HC into the exhaust gas, thenumber of feed ports for feeding HC may be made greater than the numberof feed ports feeding HC in ordinary NOx release control or ordinary PMremoval control. Due to this as well, in SOx release suppression/NOxrelease control or SOx release suppression/PM removal control, the HCfed from the HC feed valve 14 becomes able to easily diffuse in theexhaust gas, so SOx is kept from being released from the SOx trap 11.

Further, several of the NOx release control methods of the plurality ofthe above-mentioned embodiments may be combined within a range notgiving rise to any contradictions, while several of the PM removalcontrol methods of the plurality of the above-mentioned embodiments maybe combined within a range not giving rise to any contradictions.

Further, the NOx release control or PM removal control of theembodiments other than the above-mentioned second embodiment, thirdembodiment, eighth embodiment, and ninth embodiment can be applied notonly to an internal combustion engine feeding HC from the HC feed valve14 into the exhaust gas, but also to an internal combustion engineinjecting fuel from a fuel injector 3 in a latter half of an expansionstroke or during an exhaust stroke of a specific cylinder.

Note that the present invention was explained based on specificembodiments, but a person skilled in the art could make various changes,modifications, etc. without departing from the claims and idea of thepresent invention.

DESCRIPTION OF REFERENCES

-   11. SOx trap-   12. NOx catalyst-   12 a. Particulate filter-   14. HC feed valve

1. An exhaust purification system of an internal combustion engineproviding an SOx trap for trapping the SOx in the exhaust gas inside anexhaust passage, said SOx trap trapping the SOx in the exhaust gas whenan air-fuel ratio of the exhaust gas flowing into said SOx trap is anair-fuel ratio leaner than a stoichiometric air-fuel ratio and atemperature of said SOx trap is lower than a predetermined temperatureand releasing the trapped SOx when the air-fuel ratio of the exhaust gasflowing into said SOx trap is the stoichiometric air-fuel ratio or anair-fuel ratio richer than that and the temperature of said SOx trap ishigher than said predetermined temperature and executing HC feed controlfeeding HC into the exhaust gas upstream of the SOx trap when apredetermined condition stands, said exhaust purification system of aninternal combustion engine executes, as said HC feed control, first HCfeed control feeding HC into the exhaust gas upstream of the SOx trap bya predetermined pattern when the amount of SOx which the SOx trap trapsis smaller than a predetermined amount and executes, as said HC feedcontrol, second HC feed control feeding HC into the exhaust gas upstreamof the SOx trap by a pattern different from said predetermined pattern,which pattern keeping the temperature of the SOx trap from locallybecoming higher than said predetermined temperature or suppressing theformation of a region in the exhaust gas flowing into the SOx trap wherethe air-fuel ratio becomes locally rich, when the amount of SOx whichthe SOx trap traps is larger than said predetermined amount.
 2. Anexhaust purification system of an internal combustion engine as setforth in claim 1, wherein, in said first HC feed control, apredetermined amount of HC is fed into the exhaust gas upstream of theSOx trap per unit time, while in said second HC feed control, an amountof HC smaller than said predetermined amount is fed into the exhaust gasupstream of the SOx trap per unit time.
 3. An exhaust purificationsystem of an internal combustion engine as set forth in claim 1,wherein, in said second HC feed control, HC with a higher diffusionability into the exhaust gas than the HC fed into the exhaust gasupstream of the SOx trap in said first HC feed control is fed into theexhaust gas upstream of the SOx trap.
 4. An exhaust purification systemof an internal combustion engine as set forth in claim 2, wherein, insaid second HC feed control, HC is fed into the exhaust gas upstream ofthe SOx trap so that a lean degree of the air-fuel ratio of the exhaustgas flowing into the SOx trap is kept larger than a predetermined leandegree.
 5. An exhaust purification system of an internal combustionengine as set forth in claim 3, wherein, in said second HC feed control,HC is fed into the exhaust gas upstream of the SOx trap so that a leandegree of the air-fuel ratio of the exhaust gas flowing into the SOxtrap is kept larger than a predetermined lean degree.
 6. An exhaustpurification system of an internal combustion engine as set forth inclaim 4, wherein said predetermined lean degree is set larger the lowerthe temperature of the SOx trap.
 7. An exhaust purification system of aninternal combustion engine as set forth in claim 5, wherein saidpredetermined lean degree is set larger the lower the temperature of theSOx trap.
 8. An exhaust purification system of an internal combustionengine as set forth in claim 1, wherein, in said second HC feed control,HC is fed into the exhaust gas upstream of the SOx trap so that theamount of local temperature rise of the SOx trap per unit time is keptsmaller than the amount of local temperature rise of the SOx trap perunit time allowed in said first HC feed control.
 9. An exhaustpurification system of an internal combustion engine as set forth inclaim 8, wherein, in said second HC feed control, HC is fed into theexhaust gas upstream of the SOx trap so that an amount of temperaturerise of the SOx trap as a whole per unit time is kept smaller than anamount of temperature rise of the SOx trap as a whole per unit timeallowed in said first HC feed control.
 10. An exhaust purificationsystem of an internal combustion engine as set forth in claim 8, whereina particulate filter trapping particulate matter in the exhaust gas isarranged in the exhaust passage downstream of said SOx trap, one saidpredetermined condition is a burnaway condition where it is judged ifthe temperature of said particulate filter should be raised to apredetermined target temperature to burn away particulate matter trappedby the particulate filter, and, when said second HC feed control isexecuted when said burnaway condition stands, in said second HC feedcontrol, HC is fed into the exhaust gas upstream of the SOx trap usingas a target temperature a temperature lower than said target temperaturein said first HC feed control in the case where said first HC feedcontrol is executed when said burnaway condition stands.
 11. An exhaustpurification system of an internal combustion engine as set forth inclaim 9, wherein, in said second HC feed control, HC is fed into theexhaust gas upstream of the SOx trap so that a temperature amplitude ofthe SOx is kept smaller than a temperature amplitude of the SOx trapallowed in said first HC feed control.
 12. An exhaust purificationsystem of an internal combustion engine as set forth in claim 10,wherein, in said second HC feed control, HC is fed into the exhaust gasupstream of the SOx trap so that a temperature amplitude of the SOx iskept smaller than a temperature amplitude of the SOx trap allowed insaid first HC feed control.
 13. An exhaust purification system of aninternal combustion engine as set forth in claim 8, wherein, an NOxabsorbent absorbing the NOx in the exhaust gas is arranged in theexhaust passage downstream of said SOx trap, one said predeterminedcondition is an NOx release condition where it is judged that said NOxabsorbent should release NOx, and, when said second HC feed control isexecuted when said NOx release condition stands, in said second HC feedcontrol, HC is fed into the exhaust gas upstream of the SOx trap so thata temperature amplitude of the SOx trap is kept smaller than atemperature amplitude of the SOx trap allowed in said first HC feedcontrol in the case where said first HC feed control is executed whensaid NOx release condition stands.
 14. An exhaust purification system ofan internal combustion engine as set forth in claim 1, wherein anoxidation catalyst provided with an oxidizing ability higher than eventhe oxidizing ability of said SOx trap is arranged in the exhaustpassage upstream of said SOx trap.